A variety of applications for rubber and plastic compositions based on acrylonitrile-butadiene copolymers and ternary polymers require substantial reduction in the inherent flammability of the compositions. The use of organic compounds of chlorine or bromine as additives for this purpose is known (1). While these additives are effective, the amounts required to impart the desired degree of flame retardancy are frequently great enough to result in substantial detriment to other important properties of the compositions, e.g., mechanical strength (such as tensile and impact strength), modulus, deflection temperature and the like. FNT (1) J. W. Lyons, The Chemistry and Uses of Fire Retardants (Wiley--Interscience, 1970), pp. 328-332.
The use of antimony trioxide as a synergistic additive to reduce the amount of halogen compound required in these compositions is also known (2). However, antimony trioxide is costly and its recent history of limited availability and price instability has shown the hazard of reliance on a single material for this purpose. In the case of acrylonitrile-butadiene-styrene (ABS) compositions, antimony trioxide also produces undesirable side-effects on mechanical properties such as impact strength. FNT (2) Ibid.
Alternatives to the use of antimony trioxide have been reported to be applicable to nitrile rubber compositions. Thus, rubber compositions comprising either a natural rubber or a synthetic rubber, such as styrene-butadiene rubber (SBR), a reinforcing black, and a fire-retardant system consisting of a chlorine compound plus calcium borate, zinc borate and iron oxide have been disclosed in Belg. Pat. No. 766,567.
Metallic compounds which have been disclosed as fire-retardant additives for ABS include stannic oxide hydrate (3) and compounds of fluorine (U.S. Pat. No. 3,634,311). Weakly effective hydrated metallic compounds, such as hydrated alumina and zinc borate, are also known (4). Chromium carboxylates have also been disclosed (U.S. Pat. No. 3,729,436) in ABS/polyvinyl chloride blends of approximately 50/50 composition. FNT (3 ) J. Fire & Flammability 3, 130 (1972). FNT (4) Modern Plastics Encyclopedia 50 (10A) 236, 718 (1973-74).
Styrene polymers closely related to ABS have been flame-retarded (Belg. Pat. No. 643,139; Belg. Pat. No. 621,125; Ger. Pat. No. 1,282,937; Ger. Pat. No. 1,282,938) with combinations of chlorine compounds (or chlorine compounds and bromine compounds) and various metallocenes, metal carbonyls, metal dye complexes, or the like. However, these metallic additives are costly and present disadvantages such as volatility, instability or toxicity. Ferrocene, for example, which is most completely described (in the immediately preceding four patent references) in this usage, is far more costly than antimony trioxide and is volatile at elevated temperatures. Thus, it is an undesirable additive for polymers such as ABS which are processed at high temperatures. Other flame retardant systems for styrene polymers have included metal carboxylates, such as iron naphthenate, in combination with an organic chlorine compound or in combination with both an organic halogen compound and a synergistic hydrocarbon (U.S. Pat. No. 3,595,815; Fr. Pat. No. 1,520,880). The nature of these systems imposes serious limitations on the balance of cost flammability and other desirable properties obtainable. Likewise, the alternative utilization of lead compounds in these and other systems also poses toxicity hazards.
The use of metal chelates (including ferric acetylacetonate) has also been described (Brit. Pat. No. 1,047,482) in polymers containing at least 50 mole percent of styrene, but these polymers are generally unsuitable for ABS applications.
It is also known (Ger. Pat. No. 1,169,122) that iron halides, such as ferric chloride, can serve as flame retardant additives. However, the sensitivity of these materials to atmospheric moisture seriously limits their utility in polymers.
U.S. Pat. No. 3,766,157 deals with smoke-retardant polymer compositions comprising a styrene polymer and sufficient iron 8-hydroxyquinolate to reduce smoking. The use of this iron compound does not form part of the present invention.
U.S. Pat. No. 3,697,456 teaches the combination of a halogenated organic compound and ferric oxide or other oxides for flame retarding polyurethane foam. These metal oxides are partial, rather than total, replacements for antimony trioxide in polyurethane foam. This is brought out at col. 5, lines 30-33: "Thus surprisingly while the metal oxides used according to the invention are highly effective partial replacements for Sb.sub.2 O.sub.3, they are not effective when used as total replacements."